Explain The Concept Of Virtual Memory And Why It Is Needed?

An imaginary memory area supported by some operating systems (for example, Windows but not DOS) in conjunction with the hardware. You can think of virtual memory as an alternate set of memory addresses. Programs use these virtual  addresses  rather than real addresses to store instructions and data. When the program is actually executed, the virtual addresses are converted into real memory addresses.  The purpose of virtual memory is to enlarge the address space, the set of addresses a program can utilize. For example, virtual memory might contain twice as many addresses as main memory. A program using all of virtual memory, therefore, would not be able to fit in main memory all at once. Nevertheless, the computer could execute such a program by copying into main memory those portions of the program needed at any given point during execution.  To facilitate copying virtual memory into real memory, the operating system divides virtual memory into pages, each of which contains a fixed number of addresses. Each  page  is stored on a disk
until it is needed. When the page is needed, the operating system
copies it from disk to main memory, translating the virtual addresses
into real addresses.  The process of translating virtual addresses into real addresses is called mapping. The copying of virtual pages from disk to main memory is known as paging or swapping.

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A virtual memory is known as a computer system technique which is used as Systems using virtual memory technique along the RAM works more efficiently rather than the ones that works without this technique.

Virtual memory is a computer system technique which gives an impression to an application program as like it has an adjacent working memory. On the other hand it might be a possibility that an overflow on the disk storage or physical fragmentation may exists. Now a days almost all computer OS use this technique as like for normal applications that include spreadsheets, word processors, players etc. In simpler terms this virtual memory is used as to extend the memory but not by overlapping or swapping data.

Distinguish between logical and physical addresses

Virtual memory uses disk storage to extend conventional memory, or RAM. It usually works in a sequence like this: The operating system delivers programs for processing to the virtual memory area on disk. The programs generally are divided either fixed length pages or variable-length segments. A virtual memory system based on paging might break a program 400kilobytes long into ten pages of 40 kilobytes each. As the computer works on the program, it stores only a few pages at a time in RAM. As it requires other pages during program execution, it selects them from virtual memory and writes over the pages in RAM that it no longer needs.

All the original pages, or modified ones, remain intact in virtual memory as the computer processes the program. If the computer again needs a page previously in RAM but now written over, it can readily fetch the needed instructions. This process continues until the program finishes executing. Segmentation works like paging except that the lengths of segments vary. In virtual memory each block contains different instruction for each segment. Some systems combine segmentation and paging.

A virtual memory is a part of a computer which enables the user to open more than one program or perform more than one task on its computer. The virtual memory depends upon the capacity of RAM (Random Access Memory) we have on our computers. The higher the RAM, the as many tasks you can perform on your computer. If you have less RAM, the computer will show you an error and tell you to close any program in order to run a new program. The capacity of RAM depends upon the kind of operating system you are using. In case you are using Windows XP, the platform requires more than 256 megabytes of RAM. The RAM also increases the speed of the opening and closing of the programs and is available at every computer store at a quite reasonable price.

Virtual memory is a technique to execute processes that may not be completely in main memory. It abstracts main memory into an extremely large storage. This array of storage is mapped on to main memory and its backing store. The backing store is usually an area of the disk outside of the file-system.    We have also seen that:  1.    A process can be loaded in separate parts using paging.  2.    Logical addresses are translated to physical addresses at run time.  These facts indicate that it is not necessary to load complete process into memory during its execution. The portions or pages which are actually being referenced at any time must be present. The remaining process can be retained on secondary storage i.e. Hard disk.  If the flow of execution moves to a page that is not in memory, the operating system has to load the required page from secondary storage into the memory before execution can continue.    The implications of this are:  1.    Since processes can be non-contiguous in a paging system, more processes can be sustained simultaneously.  2.    Each process can be larger than the available real memory.          The programmer is freed from the limitation of containing his process within the limits of the available memory.  The programmer is presented with the illusion that computer has a large main memory to be used. It is termed virtual memory and is available for every process. The actual physical memory into which the process parts are loaded is referred to as real memory.

Vertual memory is the memory available in total on the system usually.

Virtual Memory as is the part of memory which is not physically present
but computer programmer/user can logically use it. It is generally
secondary storage memory like hard disk,CD's etc.

Virtual memory is a technique which gives an
application program the impression that it has contiguous working
memory in the computer system.

Virtual Memory is a concept in which user of a system feels to have a large physical memory(Main) that actually does not available.
For example you feel unlimited memory(Main) though you have only in MB's.